One type of vehicle drivetrain includes a main drive axle and an auxiliary drive axle that is connected to the main drive axle to provide driving upon slippage of the main drive axle. Hydraulic couplings have previously utilized Gerotor-type pumps in association with a clutch as disclosed by U.S. Pat. Nos. 5,310,388 Okcuoglu et al.; 5,536,215 Shaffer et al.; 5,595,214 Shaffer et al.; and 5,735,764 Shaffer et al. These prior hydraulic couplings include a valve that closes upon differential rotation of a pair of axle half shafts to provide piston-actuated clutch actuation while permitting a certain amount of hydraulic bleed flow.
U.S. Pat. No. 4,730,514 Shikata et al. discloses a differential mechanism wherein a hydraulic pump controls operation of a clutch and a bevel gear-type planetary gear set that extend between two rotary members such that a limited slip function of the differential gear set is provided, and there is also a relief valve that opens to suppress pressure.